1. Field of the Invention
This invention relates to record material for receiving image-wise information and is concerned more specifically with a clay-coated record material capable of forming colored images of improved durability or performance, particularly with respect to fading with exposure to light and/or ordinary atmosphere. The record material of this invention is especially formulated for use in the so-called carbonless copying or colorless "carbon paper" systems in conjunction with one of a well-known group of colorless dye-forming compounds or precursors, which cooperates with the clay-coating on the record material to produce on contact a distinct color in accordance with an electron donor-acceptor reaction mechanism which is per se well known in the art. The present record material can serve as the image-receiving sheet part of a pressure-sensitive set, in association with a transfer sheet carrying in some pressure-releasable form, e.g., within pressure-rupturable microcapsules coated thereon, the colorless dye precursor or can be used in the form of a self-contained or so-called "couplet" sheet in which the clay coating is provided on a common face of a sheet support with a colorless dye precursor in some temporarily isolated, pressre-releasable condition.
2. Description of the Prior Art
In recent years carbonless copying or colorless carbon paper systems have come into wide-spread usage for business records, print-out paper for computers and so on, and a very considerable bulk of patent literature pertaining to the preparation of such materials has grown up starting from two early series of patents U.S. Pat. Nos. 2,505,470 -489; 2,548,366-366; and 2,550,467-473. All of these patents in general disclose pressure-sensitive record material utilizing in various physical associations or arrangements a color-forming dye precursor compound, preferably colorless, selected from one of a number of chemical classes and contained in some kind of pressure-rupturable or -releasable state, in conjunction with an image-developing sheet carrying solid particles of a material reacting with the dye precursor to produce a visibly colored reaction product, such as kaolin, attapulgite and other comparable clays. The dye precursor and the reactive clay form the components of an electron donor-acceptor surface absorption chemical reaction, resulting on contact in a distinctively colored reaction product.
From the beginning, colorless copying systems operating on the basis of this donor-acceptor mechanism have been plagued with loss of coloration of the developed dye image with exposure to light and ordinary atmosphere. Such image deterioration could certainly be avoided if all office records could be maintained under high vacuum, but this is not a practical approach except in most exceptional circumstances and would be expensive and inconvenient at best. More durable images could undoubtedly be obtained by the image-wise transfer of inert pigments, such has carbon black, contained in some pressure-releasable structure. The paramount objective of a colorless or carbonless copying system, however, ruled out the possibility of using such a pigment, since the pressure-transferable surface of the resultant material would necessarily have essentially the appearance of conventional carbon paper which was aesthetically objectionable. Any system obtaining a colored image by the chemical reaction of uncolored or differently colored reactant components is inherently susceptible to color fading or deterioration by light and atmosphere and improvement in the useful like of such images is perhaps the most important single desideratum in this art.
One expedient adopted by the leading developer of colorless carbon paper systems involved judicious combination of plural dye precursor compounds, including a relatively fast reacting but relatively fugitive primary color-forming precursor compound in association with a secondary color-forming compound converting from colorless to colored state over prolonger periods of time and optimally under essentially the same conditions that accelerated the fading of the primary color-forming precursor. For example, a triphenylmethane precursor such as Crystal Violet Lactine might be combined with a leuco compound such as leuco methylene blue. Indeed, most of the patents identified above, e.g., U.S. Pat. No. 2,505,470, recognized even at that early date the desirability of aggregating two or more precursor compounds behaving in such a manner during color formation as to compensate for the inadequacies of one another. While this approach to solving the problem was an improvement, it obviously could not be considered an ideal answer. It required the provision of a full image-forming amount of each of the plural precursors, adding to the complexity and cost of the system, especially where the volume of the sheet material being produced was large, and necessitated the careful selection of precursors to achieve a compatible combination with appropriately balanced fading characteristics so that the image did not at any time pass through an essentially colorless stage. In addition, the color of the image tending to change in time, which was often disconcerting to the average user, and the range of colors that could be conveniently made available was quite limited.
In recognition of the limitations implicit in the just described approach, it has been more recently modified by adding an acidic polymeric material, particularly a phenolic resin, to the image-developing coating on the record material to react with the dye precursor rather than the clay. Such a modified system is disclosed in Canadian Patent No. 768,039 issued Sept. 26, 1967, and in British Patent No. 1,090,866 published Nov. 15, 1967, among others. While the acidic polymeric material was allegedly superior to the previous system in reducing the susceptibility of the record material to desensitization before use by exposure to ambient conditions, even in this improved system a combination of substantially oppositely behaving colorless dye precursor compounds is preferably employed. In further improvements along the lines of the acidic polymer coated record material, as in U.S. Pat. No. 3,427,180, for example, the importance from a practical standpoint of combining a rapidly developing high-fading color precursor with a slow developing secondary color precursor is still strongly emphasized. Moreover, the addition of the polymer was more costly than the clay alone, especially since some clay was included nevertheless for appearance sake, and the feel of the modified record was unnatural compared to regular paper. Obviously, therefore, the several approaches embodied in existing commercial systems do not constitute a fundamental advance in the art in overcoming or at least significantly retarding the fading of color images produced on record material by means of an electron donor-acceptor solid surface color-forming reaction mechanism.
In accordance with the present invention, a bentonite-type clay chemically modified so as to favorably influence its rheological characteristics is used as the active electron receiving-color-forming component carried on the face of a record material adapted to form an image thereon by means of the electron donor-acceptor mechanism previously referred to. The research underlying this invention has established that a bentonite-type clay is uniquely equipped in principle to serve as an electron receptor in this mechanism but is subject to the practical obstacle that a clay of this type has in the past been impossible to coat as an aqueous composition under conditions necessary for commercial production. Virtually all literature concerned with such clays record their property of solidifying at rest into a gel when mixed with water at concentrations of about 5-10% and above. Such gels are thixotropic, becoming flowable when subjected to shearing or agitating forces, and returning to the gel state promptly on removal of these forces.
Thixotropy is a desirable property for certain utilities for bentonite, among the more prominant of which is as a drilling mud for oil well drilling, but it is an insurmountable impediment to coating the material as a thin layer on a suiable record material support, nominally paper. Even if the large amounts of mechanical energy necessary to place the thixotropic gel in flowable condition were available, it would be impossible by any conventional commercial coating device, whether using an air knife, a coating blade, a coating roller or otherwise, to apply an acceptable coated layer of such a gel. Immediately on leaving the coating instrumentality, i.e., the blade or the air knife, the composition returns to its gel state, preventing the occurrence of a leveling or flowing action that is essential to produce a thin uniformly flat layer free of channelling, striations and other objectionably appearing surface effects. Even if the coating step is followed by a calendering operation, as is frequently the case, the quality of coating needed for commercial acceptance could not be achieved. Moreover, clay coating compositions for sheet material require clay concentrations substantially greater than 10% for economically feasible manufacture, in the order of at least about 20% of clay solids to avoid excessive absorption of water by the paper, causing cockling and drying problems. Ordinarily, concentrations higher even than 20% are considered desirable to reduce to as low a level as possible the amount of water that must be subsequently evaporated in order to achieve a dry coated sheet material. The power requirements to fluidize, even momentarily, an aqueous bentonite composition containing 20% or more of clay solids would be rediculously prohibitive.
The coating of paper with ordinary clays is commonplace in the art and, in fact, the paper industry is the largest consumer of kaolinite clays. It is known in this industry to adjust the rheological properties of the coating suspension or "color" by means of so-called peptizing agents, such as inorganic salts, e.g., NaCl, and the polyphosphates, alkalis, e.g., NaOH, or organic compounds, e.g., tannin and the phenolates. Such agents are not suitable for this invention, however, since at influential levels of concentration, they desensitize the clay against reaction with the dye precursor and apparently actually compete with the precursor for reaction sites on the clay molecules.
A number of patents in the field of this invention include bentonite in lists of acidic reacting clays useful in the practice of the respective contributions thereof, starting from the early U.S. Pat. No. 2,505,470 and extending through a number of more recent U.S. patents including U.S. Pat. Nos. 3,330,722, 3,389,007, 3,455,721, 3,516,845 and British patent No. 1,082,293 to mention a few. None of these patents even purport to acknowledge the over-whelming rheological considerations involved in the successful coating of bentonite clay compositions, or to discuss this type of clay except in the context of a general disclosure of equivalents and it is no accident that all of the working examples that can be found in the patents in this field which do not mention bentonite call for the use of other types of clays, typically kaolin or attapulgite. Clearly, the inventors of these patents made no attempt to deal constructively with the difficulties inherent in the handling of bentonite compositions for sheet coating purposes and, consequently, throw no light whatever on the actual practical use of this particular type of clay mineral.